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    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
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<body>
    
</body>
<script>
    let num1; num2 = 0, num3 = 123, str1 = '', str2 = 'hello', flag = null;
    console.log(Boolean(num1));//未定义型转布尔型：false
    console.log(Boolean(num2));//数值0转布尔型：false
    console.log(Boolean(num3));//非零非数值转布尔型：true
    console.log(Boolean(str1));//空串转布尔型：false
    console.log(Boolean(str2));//非空字符转布尔型：true
    console.log(Boolean(flag));//null转布尔型：false
    //
    //
    //未定义转数值
    console.log('----------------');
    console.log(Number(num1));
    console.log(parseInt(num1));
    console.log(parseFloat(num1));
    console.log('---------------');
    //字符型转数值
    console.log(Number(str1));//空串转数值
    console.log(parseInt(str1));//空串转数值
    console.log(parseFloat(str1));//空串转数值
    console.log(Number(str2));//非空串转数值
    console.log(parseInt(str2));//非空串转数值
    console.log(parseFloat(str2));//非空串转数值
    let str4 = '4.34hello';
    console.log(Number(str4));
    console.log(parseInt(str4));
    console.log(parseFloat(str4));
    console.log('---------------');
</script>
</html>